Alkali metal hydroxides can be prepared using different types of electrolytic cells, including membrane and diaphragm-type cells. The present invention is particularly directed to a method of making caustic using a diaphram electrolytic chlorine cell, and more specifically for a chlor-alkali processes.
After electrolysis, the caustic stream is traditionally treated to become more concentrated. During that process, the caustic often reacts with the reactors which are typically made of nickel causing numerous problems in the process.
A need has long existed in the caustic industry for a technique to remove metal, particularly nickel and iron from a caustic manufacturing process without adding additional chemicals to the process.
A need has existed for an environmentally friendly metal removal process which lowers the cost to manufacture at least two grades of caustic, standard and bleach grade.
Alkali metal hydroxide solutions are typically prepared by the electrolysis of alkali metal salt solutions in electrolytic cells. Diaphragm cells are known for producing alkali metal hydroxide solutions electrolytically. In this type of cell, a porous cathode with an overlying porous diaphragm is used to separate or to serve as a barrier between the catholyte and anolyte compartments of the cell. An objectionable feature of this system is that sodium borohydride has been needed to be added to the solution to reduce the effect of nickel on the further treatment of caustic. The use of the sodium borohydride has caused high expense and increased the presence of nickel oxides in the reactors.
The present invention is directed at a method for reducing the amount of free nickel and nickel oxides, free iron and ferrous oxides in the chlor-alkali systems, particularly for the diaphragm types of cells without the need for additional chemicals, additional sodium borohydride, or other contaminants.
In order to overcome the disadvantages of the prior art the present invention proposes the use of magnets in situ in the caustic manufacturing process to overcome these problems with the nickel and the iron.